The year is 196303 BC, and a rough band of cavemen scrambles from the mud in a desperate search for food. After two failed attempts to fill their bellies, hunger and fear push the group toward a desperate decision: they narrowly consider cannibalism, a grim option that underscores how thin their margins are. When women appear, the culture shock is clear, and the elder among them sees something unexpected in the behavior of a pair of stegosauruses. The sight sparks a strange, primal idea to imitate that mating display. From this strange spark, a pair of pregnancies emerge, and nine months later a woman carries two boys, Ah and O. The group’s reactions are telling: they’re drawn to Ah, yet a mixture of fear and superstition drives them to abandon O. O becomes the child who will carry a long, tumultuous journey.
A brontosaurus also plays a crucial part in this origin story, abandoning one of her eggs, a detail that will ripple forward through the lives of every creature introduced here. From the egg hatches a baby brontosaurus named Igua, and he and O form an immediate, if uneasy, bond as friends. As the years pass, O’s hunger and curiosity push him to stay alive and explore, and a chance encounter with a stranded egg leads to another new companion: a baby pterodactyl named Croak. Croak enters O’s world, and the two quickly become friends, even as Igua grows wary and jealous, unable to accept a closeness that eclipses his own place in the trio.
O grows into an adventurous young figure, and Croak’s loyalty is tested as their bond tightens. Croak tries to mentor O, teaching him the basics of being more than a mere animal. Their first “lesson” is the dream of flight, a task that proves difficult and dangerous. When O attempts to fly, Igua intervenes, pulling him away and reigniting their competing loyalties. A reflective moment follows: O sees his own reflection in a pool and realizes that he is not a brontosaurus anymore, but a man. This discovery nudges him toward leaving Igua behind to seek his own place among humans.
The jungle becomes O’s classroom as he encounters creatures that challenge his assumptions about what makes a man. He mistakes several animals for human rivals—first a wild boar, then a colossal worm—before getting lost in the encroaching darkness of night and narrowly escaping a menagerie of perilous beasts. In the midst of this chaos, Croak reappears, and the two set out again, finally taking to the skies together. A misstep during flight leads to a separation when Croak crashes into a dimorphodon, throwing O’s world into a new orbit of danger and wonder.
Meanwhile, Igua’s quest to join his own kind intensifies. He finds a lake crowded with brontosauruses who sniff out his “man scent” and quickly reject him. Their rejection pushes Igua to chase after O, convinced that only by finding the world of humans can he belong. O’s wanderings bring him into contact with a curious, industrious group known as the No-Lobes. They tolerate him initially, but O’s mischief soon proves costly for their crops, and he’s cast out as further misfortune unfolds. During this wandering, O encounters a feline tribe whose women use long tails to seduce their prey, only to kill them afterward. O is captured by No-Lobes, but fate twists in him in unexpected ways.
The feline tribe’s most alluring danger is No-man, a cunning temptress who clutches the power of her tail to lure O away to her own kind. A newborn baby distracts the felines, giving O and No-man a chance to flee and, in a tense sequence, they manage to consummate their uneasy relationship. The escape is chaotic: a stampede of giant turtles threatens to overwhelm them, and the feline hunters close in. O improvises a daring rescue using a lone giant turtle shell and wheels to reach No-man, but the effort is incomplete, and O rolls off toward the desert, leaving No-man’s tail to be cut off in the chase.
Desert solitude follows, a brutal test that ends with a crash into a palm tree and a long walk ahead for O. Fate, however, soon nudges him toward Croak again. The two reunite by a water source that looks like life but is guarded by danger. A dragon—voiced by Bill Murray—blocks the water and becomes part of the strange, comic trials that weave through O’s journey. O’s cleverness comes into play when he helps the dragon by corking the dragon’s rear end, a trick that makes the dragon breath fire from its mouth instead of elsewhere. The dragon then allows O to drink, but the watering hole proves to be a tar pit. The dragon, nonetheless, provides a lift to a nearby lake, where O quenches his thirst and rests, only to discover that he’s entangled in a new set of trials.
Free of the tar pit, O encounters a bold Arctic trek where a group of Norwegian barbarians marvel at fire. They use fur coats to intensify the flame, but the heat destabilizes their world and eventually leads to a fall into the ocean’s depths. Croak arrives just in time to save O, and as the duo escape, a shark interrupts their flight and nearly ends O’s journey. Yet O survives, and the long road toward his human family resumes, and this time he finally meets them again.
The elder among O’s newly found family recognizes him—and, in a sudden twist of fate, dies. O steps into leadership and looks to teach his new world what he has learned on his travels. Yet Igua catches up, and the brothers’ long-awaited reunion sits on a knife-edge. Ah, O’s perceived rival and brother, seizes leadership for a moment and hurls O into the fire that burns in the heart of their world. Igua intervenes to save him, and the two reconcile their friendship amid the chaos of prehistoric conquest.
Ah’s ambition drives him to declare a world-altering crusade: they must reshape Earth in their image and eliminate any creature that stands in their way. The result is catastrophic. The dragon vanishes from the map, the No-Lobes face a brutal extermination in a war spurred by Ah, the dinosaurs rush toward extinction through mass suicide, the feline clan falls to bloodshed and slavery (with No-Man surviving only to part ways with O), and the ant colony makes a quixotic attempt to escape via a grass rocket that spectacularly fails and triggers the splitting of Pangaea into the Earth’s continents. In the end, O, Croak, and Igua escape the devastation and settle on a deserted island, where O adopts the title of the missing link and contemplates the weight of his actions as the world carries on.
